How to carry out an gearbox pressure test on a small outboard engine.
You will need a pressure testing kit
And an Adapter to go from the tube fitted to your pressure testing tool to the outboard engine gearbox. The gear lube pump below contains 2 adapters which will cover the vast majority of gearboxes, the 8mm adapter in the quicksilver pump is good for most outboard gearboxes but the yellow 3/8 plastic one isn’t up to too much for use directly in the gearbox so if you have a Johnson, Evinrude, US Mercury etc which have the 3/8 plug size, consider buying the Performance tool 3/8 marine lower fill adapter on the right, they really are good.
First you need to make sure you have the correct tools to carry out the pressure test. Take the adapter that fits into your gearbox fill or level screw and attach this to the pressure testing kit using a small jubilee clip to make sure you have an airtight seal.
You will end up with something very similar to this
There is no need to remove the lower unit/gearbox to carry out the test but if it fails you will most certainly need to remove it if you want to carry out the repairs yourself.
Remove the gearbox fill/level screws and drain the gearbox oil.
Refit the upper level plug with a sealing washer in good condition and tighten it up
Screw your adapter into the lower drain plug and hand tighten it.
Using the hand pump on your tool, pressurise the gearbox to 10 psi and monitor for 10 mins, in that period there should be no drop of pressure but don’t worry if it only drops 1 psi or so over 10 mins.
IF the gauge shows a drop in pressure of more than 1 psi or so, mix up a small pot of washing up liquid and water at about a 75-25 ratio. Using a small paint brush, brush the liquid around anywhere that could possibly leak (including your pressurising tool connections) and re-pressurise, leaks will be revealed as bubbles forming.
This is a typical leaking gearbox input seal with the liquid mix brushed on there and pressure in the gearbox, you can see bubbles forming where the driveshaft passes through the input seal so the seal is leaking and needs replacing.
If the pressure holds just fine, you have carried out a gearbox pressure test on a small outboard engine, remove your test kit, refill the gearbox and you are good to go, if there are leaks you can’t identify or repair yourself, seek professional help.